DC-DC converter, as a device for realizing conversion of different DC voltages using a switching technique, is widely used in consumer electronics and industry electronics, and can increase power efficiency of a battery effectively. Therefore, conversion efficiency is a key parameter to measure performance of the DC-DC converter. In addition, when the DC-DC converter is desired to output a lower voltage, e.g., when it is used for supplying power to a memory, ripple of the output voltage is also a very important performance parameter.
Generally speaking, in order to acquire high conversion efficiency, different control methods are adopted for a heavy load and a light load, respectively. At the heavy load, the DC-DC converter usually operates in a continuous mode, while at the light load, the DC-DC converter usually operates in a discontinuous mode. These two modes correspond to two non-overlapping load regions. If the load regions overlap with each other, the two modes will be switched frequently in the case of a certain load, so as to output a larger ripple. As a result, it is required to design an effective mode switching method, so as to ensure that the load regions of the two modes do not overlap with each other, thereby to decrease the ripples of the output voltage.
As shown in FIG. 1, a DC-DC converter includes a first switch 102, a second switch 103, an inductor 104, a capacitor 105 and a controller 107.
A power source 101 is connected to the ground via the first switch 102 and the second switch 103 that are connected in sequence.
The inductor 104 and the capacitor 105 that are connected in sequence have one end connected between the first switch 102 and the second switch 103 and the other end connected to the ground.
A load 106 has one end connected between the inductor 104 and the capacitor 105 and the other end connected to the ground.
The controller 107 controls the first switch 102 and the second switch 103 so as to enable the converter to operate in different modes when the load 106 is in a light load state or in a heavy load state, thereby to increase the power conversion efficiency.
At the discontinuous mode, the DC-DC converter has two durations, i.e., active duration and inactive duration. At the active duration, the capacitor 105 can be charged continuously by controlling the first switch 102 and the second switch 103 until an output voltage Vout reaches a first predetermined threshold, and then the converter enters the inactive duration. in the inactive duration, the capacitor 105 is discharged until the output voltage Vow decreases to a second predetermined threshold (which is less than the first predetermined threshold), and then the converter enters the active duration.
The above-mentioned processes are repeated until the DC-DC converter is desired to be switched to the continuous mode.
During the switching process of the operation modes, the controller 107 shall ensure that the operation modes of the DC-DC converter will not be switched frequently in the case of a constant load.
In the prior art, when the DC-DC converter operates in the discontinuous mode, the output voltage will decrease if the load is greater than the load region corresponding to the discontinuous mode. The controller 107 compares the output voltage Vout with a reference voltage Vref. When Vout<Vref−Δ, it is judged that it needs to execute mode switching control so that the DC-DC converter is switched from the discontinuous mode to the continuous mode, Δ is a voltage used to add hysteresis to the switching from the discontinuous mode to the continuous mode, in order to prevent frequent mode switching in the case of a constant load.
It can be found from the above description that, in an existing algorithm for mode switching control from the discontinuous mode to the continuous mode, it needs a hysteresis value Δ. This value may not be too small. The output voltage will also decrease by Δ1 when the load within the load region corresponding to the discontinuous mode, thus Δ shall be greater than Δ1. This value may not be too big either, otherwise large ripples will occur when switching. In an analogue process, the value of Δ may be variable in a large range along with the change of process corner, temperature and power voltage, thus it is difficult to ensure the reliability of mode switching. In addition, in the prior art, an analogue comparator is required so as to compare two analogue values, i.e., Vout and Vref. However, the analogue comparator is constrained by semiconductor processes. For example, when the process changes from 130 nm to 40 nm, the analogue comparator needs to be redesigned, and thus the design time will be delayed.